Technologies regarding optical integrated circuits, in which optical components are integrated together, such as transistor integrated circuits integrating electronic components, have been developed. At present, optical circuits are composed of optical components such as optical switches, wavelength filters, 3 dB couplers (optical couplers), which are connected together via optical waveguides such as optical fibers, wherein it is possible to significantly reduce volumes of optical circuits, power consumption, and manufacturing cost if a plurality of optical components is integrated into a small chip. Until now, various technologies regarding optical integrated circuits have been developed, wherein Japanese Patent Application Publication No. 2002-303836, for example, discloses an optical switch having a photonic crystal structure. Photonic crystals (or “photonic crystalline”) is a general term regarding the structure undergoing periodical variations of refractive indexes of light.
Photonic crystals demonstrate various special features of optics owing to periodically structured refractive-index profiles, wherein one exemplary feature is a photonic band gap (PBG). Photonic crystals are able to transmit light therethrough, but they do not transmit light of a specific frequency band when photonic crystals undergo significantly large periodical variations of refractive indexes. The frequency bands (or wavelength ranges) of light transmitted through photonic crystals are called photonic bands. The frequency bands of light not transmitted through photonic crystals are called photonic band gaps (PBG) since they emerge in gaps between photonic bands. In some structure of photonic crystals, photonic band gaps overlap with a plurality of frequency bands. Photonic bands which are split via photonic band gaps are called a first band, a second band, and a third band aligned in ascending order of frequency.
When photonic crystals develop micro defects destroying periodically structured refractive-index profiles (or periodicity of refractive indexes), light having frequencies of photonic band gaps are confined in micro defects. In this case, light whose frequency depends on the sizes of micro defects are solely confined in micro defects; hence, those photonic crystals serve as resonators. For this reason, photonic crystals can be used as frequency selective (wavelength selective) filters (or optical filters).
When a plurality of micro defects is consecutively developed in lines so as to form line defects in photonic crystals, light having frequencies of photonic band gaps is confined in line detects, so that light may propagate along line defects. That is, photonic crystals containing line defects can be used for optical waveguides. Optical waveguides composed of photonic crystals containing line defects are called line-defect waveguides.
It is possible to constitute optical functional elements such as optical modulators and optical switches by using either optical filters or optical waveguides or by combining both of them. That is, it is possible to constitute optical circuits by forming and connecting optical functional elements in photonic crystals. For this reason, it is expected that photonic crystals contribute to the platform of optical integrated circuits.
In actuality, the following limitations are applied to photonic crystal structures, which are expected to serve as the platform of optical integrated circuits.
To utilize effects of photonic band gaps in a three-dimensional manner consisting of X-axis, Y-axis and Z-axis, refractive-index profiles of photonic crystals need to be periodically structured in a three-dimensional manner. Since three-dimensionally structured refractive-index profiles are complex and entail high manufacturing cost, two-dimensionally structured refractive-index profiles have been frequently used in photonic crystals (hereinafter, referred to as photonic crystals). Actually utilized two-dimensional photonic crystals are formed in substrates with finite thicknesses, wherein refractive-index profiles indicate periodicity in a plane, but refractive-index profiles do not indicate periodicity in the thickness direction of substrates. In this case, it is hard to achieve optical confinement in the thickness direction of substrates based on the PBG effect, but it is possible to achieve such optical confinement based on the total reflection owing to refractive-index differences.
Optical features of two-dimensional photonic crystals with finite thicknesses do not perfectly match with optical features of two-dimensional photonic crystals with infinite thicknesses. When refractive-index profiles of two-dimensional photonic crystals with finite thicknesses are created with symmetry of reflection in the thickness direction in light propagating regions, their optical features approximately match with optical features of two-dimensional photonic crystals with infinite thicknesses. The operational prediction of two-dimensional photonic crystals with infinite thicknesses is by far easier than the operational prediction of two-dimensional photonic crystals with finite thicknesses. For this reason, it is possible to easily design devices using two-dimensional photonic crystals demonstrating refractive-index profiles with symmetry of reflection
At present, various structures have been developed with respect to two-dimensional photonic crystals with finite thicknesses. For instance, pillar-type tetragonal photonic crystals demonstrate features (i.e. low group-velocity characteristics) in which the speed of light propagating through line-defect waveguides decreases in a broad range of frequency bands. Generally speaking, it is possible to constitute optical circuits of predetermined functions with short waveguide lengths by using waveguides which in turn decrease light propagating speeds. Therefore, line-defect waveguides composed of pillar-type tetragonal photonic crystals are suitable to optical integrated circuits.
FIG. 12 is a perspective view showing the structure of a line-defect waveguide composed of a pillar-type tetragonal photonic crystal 100 with a finite thickness. The pillar-type tetragonal photonic crystal 100 is composed of a low dielectric material 101, which arrange cylinders 102, each having a finite height and composed of a high dielectric material, and cylinders 103 whose diameters are smaller than those of cylinders 102 in a tetragonal lattice array. The name “photonic crystals” derives from the fact that the tetragonal lattice array for arranging the cylinders 102 and 103 is likened to the lattice array of atoms in silicones and crystals and available to optical applications. In this connection, the low dielectric material 101 and the cylinders 102, 103 are not necessarily composed of crystalline-structured materials but can be composed of amorphous-structured materials.
In FIG. 12, the cylinders 102 are cylinders composed of photonic crystals with perfect periodicity, whilst the cylinders 103 are smaller than the cylinders 102 in diameter; hence, the cylinders 103 can be regarded as defects occurring in perfect crystals. The following description distinguishes between the cylinders 102 composed of perfect crystals and the cylinders 103 serving as defects, wherein the former one will be referred to as “non-line-defect cylinders” and the latter one as “line-defect cylinders”. Herein, it is noted that the line-defect cylinders 103 do not cause defects by itself.
The pillar-type tetragonal photonic crystal 100 linearly arranges the line-defect cylinders 103 therein, thus forming line-defect waveguides using the linear arrangement of the line-defect cylinders 103 and their surrounding non-line-defect cylinders 102. In the line-defect waveguide composed of the pillar-type tetragonal photonic crystal 100, the linear arrangement of the line-defect cylinders 103 is deemed equivalent to the core of a total-reflection confinement waveguide such as an optical fiber, while an array of the non-line-defect cylinders 102 on opposite sides as well as the surrounding low dielectric material 101 is deemed equivalent to the clad. Similar to the total-reflection confinement waveguide which implements a waveguide function with the core and the clad, the line-defect waveguide implements a waveguide function with the line-defect cylinders 103 and the surrounding non-line-defect cylinders 102 as well as the low dielectric material 101. The line-defect waveguide may serve as a single-mode waveguide solely indicating a basic mode when it is appropriately designed and manufactured.
As described above, wavelength filters can be constituted using photonic crystals with micro defects formed therein. In actuality, however, wavelength filters need to be accompanied with structures for inputting/outputting light. For instance, wavelength filters can be constituted by arranging line-defect waveguides proximate to micro resonators as disclosed in Japanese Patent Application Publication No. 2004-295113 or by partially inserting micro resonators into line-defect waveguides.